Conventional non-continuous fluid level sensors used in aircraft fuel systems typically are based on two-wire thermistors or float switches. Conventional thermistor-type aircraft fuel level sensors monitor the temperature change of the sensor as power is applied to the sensor. If the sensor is immersed in the fuel, the temperature will not change significantly, as the fuel acts as a heat sink. However, if the sensor is not immersed in the fuel, the power applied to the thermistor causes the sensor temperature to rise, thereby signaling that the sensor is not immersed in the fuel.
Recently, the Federal Aviation Administration has enacted Special Federal Aviation Regulation Number 88 (SFAR 88). SFAR 88 requires certain Type Certificate and Supplemental Type Certificate (STC) holders to conduct a system safety review of fuel tank systems on transport category airplanes. The above described thermistor fuel level sensor is not compatible with SFAR 88.
Float switches are another type of conventional non-continuous fluid level sensor. A float switch relies upon a magnet in a float that rides along a vertical rod and activates a reed switch in the rod at the desired fluid level (set point). This type of sensor is based upon mechanical motion and is prone to reliability problems and limited operating life.
Fluid level sensors have been used for other level sensing applications. However, these sensors typically are multi-transducer systems that utilize three or four wire configurations, which are not compatible with existing two-wire level sensors used in aircraft fuel tanks.
Further, conventional single-transducer ultrasonic sensors usually depend on a minimum propagation time of the ultrasonic wave in the fluid, resulting in a relatively long sensor and large minimum sensing height (e.g., around two to four inches above the bottom of a liquid container or tank). Such sensors rely on an ultrasonic wave traveling in a fluid reflecting off a target surface. Thus, changes in the target surface (e.g., due to contamination, degradation, etc.) can degrade sensor performance.
By way of further example, U.S. Pat. No. 4,320,659 to Lynnworth discloses an ultrasonic sensor that generates vertically polarized shear-mode (SV) ultrasonic waves, which interact with surrounding fluids through the length of the ultrasonic propagation path. The sensor relies on the SV waves “leaking” into the surrounding fluid.
U.S. Pat. No. 4,325,255 to Howard discloses a two-transducer system (e.g., one transducer for transmitting and the other transducer for receiving the ultrasonic wave) mounted in a sensor head and using a closed loop servo system for measurements.
U.S. Pat. No. 4,614,115 to Pelletier discloses a sensor based primarily on the use of non-longitudinal waves (shear waves) generating multiple (secondary) reflections off the lateral walls of a sensing rod immersed in material to be tested. These secondary waves propagate outside of the lateral walls of the sensor rod and interact with the surrounding medium. Pelletier is primarily concerned with the change in amplitude of the non-longitudinal secondary reflections over time (as a measure of change in properties of the surrounding system).
U.S. Pat. No. 4,890,490 to Telford uses Lamb waves (flexural waves) propagating in a thin metal plate (waveguide). These relatively slow, low frequency Lamb waves are designed to travel relatively long distances (tens of meters) in their sheet metal wave guide and exhibit a change in amplitude at discontinuities when fluid is present/absent.